Abstract

During prolonged running, the magnitude of Achilles tendon (AT) length change may increase, resulting in increased tendon strain energy return with each step. AT elongation might also affect the magnitude of triceps surae (TS) muscle shortening and shortening velocity, requiring greater activation and increased muscle energy cost. Therefore, we aimed to quantify the tendon strain energy return and muscle energy cost necessary to allow energy storage to occur prior to and following prolonged running. 14 trained male (n = 10) and female (n = 4) distance runners (24±4 years, 1.72±0.09 m, 61±10 kg, [Formula: see text] 64.6±5.8 ml•kg-1•min-1) ran 90 minutes (RUN) at approximately 85% of lactate threshold speed (sLT). Prior to and following RUN, AT stiffness and running energy cost (Erun) at 85% sLT were determined. AT energy return was calculated from AT stiffness, measured with dynamometry and ultrasound and estimated TS force during stance. TS energy cost was estimated on the basis of AT force and assumed crossbridge mechanics and energetics. Following RUN, AT stiffness was reduced from 328±172 N•mm-1 to 299±148 N•mm-1 (p = 0.022). Erun increased from 4.56±0.32 J•kg-1•m-1 to 4.62±0.32 J•kg-1•m-1 (p = 0.049). Estimated AT energy return was not different following RUN (p = 0.99). Estimated TS muscle energy cost increased significantly by 11.8±12.3 J•stride-1, (p = 0.0034), accounting for much of the post-RUN increase in Erun (8.6±14.5 J•stride-1,r2 = 0.31). These results demonstrate that a prolonged, submaximal run can reduce AT stiffness and increase Erun in trained runners, and that the elevated TS energy cost contributes substantially to the elevated Erun.

Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1. Experimental protocol.

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On the first…

Fig 1. Experimental protocol.

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On the first visit to the lab ( Visit 1 ),…

Fig 1. Experimental protocol.

On the first visit to the lab (Visit 1), the subject’s VO2max and 85% sLT were determined. The subjects returned to the lab 48–96 hours later (Visit 2), when AT stiffness and Erun at 85% sLT were determined before and after a 90 min run at a prescribed heart rate associated with 85% sLT.

Fig 5. Relationship between the change in E run and AT stiffness following 90 min…

Fig 5. Relationship between the change in Erun and AT stiffness following 90 min run.

The solid line represents the linear relationship between Erun and stiffness (r2 = 0.430, p = 0.011). Dashed lines represent the 95% confidence interval for the relationship. Note that the relationship crosses the abscissa at 0% change in stiffness.

Fig 6. Relationship between the change in…

Fig 6. Relationship between the change in E run and TS muscle energy cost following…

Grant support

JRF was supported by Natural Sciences and Engineering Research Council of Canada (NSERC PGSD2-391898-2010), and by Mitacs and Own the Podium through the Mitacs-Accelerate Program. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.